Baking pan or roasting pan at least 2 inches deep and large enough to hold the cake pan with at least 2 inches to spare on each side.
Position the rack in the lower third of the oven and preheat to 350 degrees. Line the bottom and sides of the pan with foil. Put a kettle of water on to boil for the bain marie when baking in the oven.
Place chocolate in a large mixing bowl. Combine the 2 egg yolks with vanilla in a small bowl. Place the 3 egg whites in a medium bowl with cream of tartar. Set all 3 bowls aside.
Combine the cocoa, flour and 2/3 cup sugar in a 1 1/2 quart heavy bottomed saucepan. Whisk in enough of the milk to form a smooth paste. Mix in the remaining milk. Cook over medium heat, stirring constantly with a wooden spoon, to prevent burning (especially around the bottom edges of the pot), until mixture begins to simmer. Simmer very gently, stirring constantly, for about 1 1/2 minutes, or until the mixture visibly thickens. Pour the hot mixture immediately over the chopped chocolate. Stir until chocolate is completely melted and smooth. Whisk in egg yolk mixture. Set aside.
Beat the egg whites and cream of tartar at medium speed until soft peaks form. Gradually sprinkle in the remaining 1/4 cup of sugar, beating at high speed until stiff but not dry. Fold a quarter of the egg whites into the chocolate mixture to lighten it. Fold in remaining egg whites. Scrape mixture into the cake pan and spread evenly. Set cake pan in baking pan and place on oven rack. Pour enough boiling water into the baking pan to come a third to halfway up the sides of the cake pan.
Bake for exactly 20 minutes. The cake will only be about 3/4 inches tall. The surface will spring back when gently pressed but cake will still be quite gooey inside. Remove cake pan and water pan from oven. Remove the cake pan from the water and cool completely on a rack. Cover with plastic wrap and refrigerate overnight before serving. Refrigerate up to 2 days or freeze up to 2 months.
After the truffle sheet is well chilled, lift the edges of the foil to remove the truffle sheet from the pan. Cut the sheet into 7 strips, each about 1 1/4 inches wide, using a long thin sharp knife dipping in hot water and wiped dry between each cut (the cake is very sticky) or cut with dental floss. Cut each strip into 7 squares. Transfer each square to a fluted paper candy cup and or arrange on a serving platter. If garnishing with raspberries, simmer the preserves for a minute or until thick and sticky. Touch the hot preserves with the bottom of each berry before placing it on the truffle square. Refrigerate until serving.